The invention relates generally to a method and system for making optical measurements, and in particular, to a method and system for making optical measurements for a catheter-based imaging system such as Optical Coherence Domain Reflectometry (OCDR).
Ultrasound medical imaging is often used to produce images of blood vessels and surrounding tissue. To image a blood vessel and surrounding tissue, an Intravascular Ultrasound (IVUS) catheter is typically used. The IVUS catheter comprises an elongated member and an ultrasound transducer located at a distal end of the elongated member. The elongated member is inserted into the blood vessel, and the ultrasound transducer is positioned at a desired location in the blood vessel. An ultrasound transducer is designed to transmit a specific resonant frequency, e.g., when it is excited by a pulse. The excite pulse signal causes the ultrasound transducer to emit ultrasound waves in the blood vessel. A portion of the emitted ultrasound waves is reflected back to the ultrasound transducer at tissue boundaries in the blood vessel and the surround tissue. The reflected ultrasound waves induce an echo signal at the ultrasound transducer. The echo signal is transmitted from the ultrasound transducer to an ultrasound console, which typically includes an ultrasound image processor and a display. The ultrasound console uses the received echo signal to image the blood vessel and the surrounding tissue.
In order to produce a radial cross-sectional image of a blood vessel and surrounding tissue, the ultrasound transducer is typically rotated along the axis of the elongated member. As the ultrasound transducer is rotated, the ultrasound transducer emits ultrasound waves in different radial directions. The resulting echo signals from the different radial directions are processed by the ultrasound console to produce a radial cross-sectional image of the blood vessel and the surrounding tissue. Alternatively, the ultrasonic transducer may be mounted in an assembly together with a reflective member (mirror), where the transducer emits ultrasonic energy in a substantially axial direction and the mirror is oriented to deflect the emitted ultrasonic energy in a radial direction.
The echo signal is a serial amplitude modulated signal, in which the amplitude of the signal varies with time. A typical echo signal has a time length of 8 xcexcs, which corresponds to an image depth of approximately 6 millimeters from the ultrasound transducer. The echo signal carries both image brightness information and image depth information, where depth may be taken with respect to the ultrasound transducer. The image brightness information is provided by the amplitude of the echo signal. The image depth information is provided by the time position within the echo signal. For example, an earlier time position in the echo signal corresponds to a lower image depth than a later time position in the echo signal. This is because an ultrasound wave that is reflected back to the ultrasound transducer from a shallower depth reaches the ultrasound transducer before an ultrasound wave that is reflected back to the ultrasound transducer from a deeper depth. As a result, the ultrasound wave that is reflected back to the ultrasound transducer from the shallower depth has a shorter propagation delay time, which translates into an earlier time position in the echo signal.
Another imaging technique used to produce images of blood vessels and surrounding tissue is Optical Coherence Domain Reflectometry (OCDR). To image a blood vessel using OCDR, a fiber-optic catheter is inserted into the blood vessel. A proximal end of the fiber-optic catheter is coupled to an OCDR system. In the OCDR system, a laser generates a source beam. A beam splitter splits the source beam into a reference beam and a sample beam. The reference beam is diffracted by a diffraction grating into a diffraction beam. The sample beam is transmitted through the fiber-optic catheter and emitted in the blood vessel at a distal end of the catheter. Typically, the distal end of the catheter includes a prism for directing the sample beam into the blood vessel. A portion of the sample beam is reflected back to the distal end of the catheter by the blood vessel and the surrounding tissue. The reflected sample beam is transmitted to the OCDR system through the fiber-optic catheter. In the OCDR system, the reflected beam is mixed with the diffraction beam to produce a coherence-domain interference pattern, which is detected by an array of photo detectors.
The resulting interference pattern provides both image brightness information and image depth information, where depth may be taken with respect to the distal end of the catheter. The image brightness information is provided by the light intensity of the interference pattern. The image depth information is provided by the spatial position within the interference pattern. This is because the portion of the sample beam that is reflected back to the catheter from a certain depth in the body constructively interferes with the diffraction beam at a certain spatial position. Typically, the photo detectors of the photo array are arranged so that each photo detector detects the light intensity of the interference pattern at a certain spatial position within the interference pattern. Thus, the output of each photo detector provides image brightness information for a certain image depth. The photo array outputs parallel channels, where each parallel channel corresponds to the output of one of its photo detectors. The parallel channels of the photo array are inputted to an OCDR image processor to produce an image of the blood vessel and the surrounding tissue.
An advantage of the above-described OCDR system is that the array of photo detectors is able to capture image brightness information at multiple image depths in one instance. This enables the OCDR system to produce images at true video rates, e.g., 30 frames per second.
Optical coherence tomography (OCT) is an optical imaging technique, which achieves non-invasive, cross-sectional imaging of microscopic biological structures. OCT is analogous to ultrasound imaging, only measuring the intensity of backscattered infrared light rather than sound. It can be implemented using high-speed fiber optics, which makes OCT compatible for interfacing with fiber optic endoscopes used for catheter-based imaging within the vascular system.
Mechanical OCT systems use a mirror mounted to a piezoelectric material to achieve mechanical scanning of depth information. However, the speed of these systems are limited by mechanical factors such as mass, the electromechanical properties of the piezoelectric material and the need to scan at a constant velocity. These limitations translate into scanning rates that are considerably slower than true video rate, such that real time viewing cannot be achieved.
In grating generated OCT systems, depth information is spatially translated across a beam of light using a diffraction grating that functions as a series of stepped mirrors. Each xe2x80x9cmirrorxe2x80x9d interacts spatially with light reflected from a sample producing information from multiple points of depth. The parallel depth information is then captured electronically with an array of parallel photoelements. Unlike mechanical OCT systems, grating generated OCT systems can achieve video rates if fast electronic processing is used. However, grating generated OCT systems require a large number of parallel electronic channels, which are very large, complex and power consuming.
Therefore, there exists a need for an OCT system that utilizes a single electronic channel yet still achieves video rate scanning speeds.
The invention relates to an OCDR system that outputs a detector signal which carries image depth information on multiple modulation frequencies, where each modulation frequency preferably corresponds to a different image depth.
A first, separate aspect of the invention is an OCDR system which outputs a detector signal which carries image depth information on multiple modulation frequencies, where each modulation frequency corresponds to a different image depth.
A second, separate aspect of the invention is an OCDR system which outputs a detector signal which carries image depth information on multiple modulation frequencies such that the image depth information from the detector signal may be resolved by tuning to the desired modulation frequency.
A third, separate aspect of the invention is an OCDR system which has a combined signal having a plurality of modulated frequency signals, where each modulated frequency signal corresponds to a different aspect of the image.
A fourth, separate aspect of the invention is an OCDR system which has a combined signal having a plurality of modulated frequency signals, where each modulated frequency signal corresponds to a different image depth.
A fifth, separate aspect of the invention is an OCDR system which allows a combined beam to focus onto a single light detector.
A sixth, separate aspect of the invention is an OCDR system which enables the use of avalanche mode photodiodes.
A seventh, separate aspect of the invention is an OCDR system which has an interferometer with multiple modulators having no transition from fiber optic to free space.
An eighth, separate aspect of the invention is a method for imaging body tissue comprising the steps of frequency modulating image depth information onto a signal and outputting the signal
A ninth, separate aspect of the invention is a method for imaging body tissue comprising the steps of frequency modulating image depth information onto a signal, where each modulation frequency corresponds to a different image depth, and outputting the signal.
A tenth, separate aspect of the invention is a method for imaging body tissue comprising the step of providing a combined signal having a plurality of modulated frequency signals, where each modulated frequency signal corresponds to a different aspect of the image.
An eleventh, separate aspect of the invention is a method for imaging body tissue comprising the step of providing a combined signal having a plurality of modulated frequency signals, where each modulated frequency signal corresponds to a different image depth.
A twelfth, separate aspect of the invention is a method for imaging body tissue which allows a combined beam to focus onto a single light detector.
A thirteenth, separate aspect of the invention is a method for imaging body tissue which enables the use of avalanche mode photodiodes.
A fourteenth, separate aspect of the invention is a method for imaging body tissue which uses multiple modulators having no transition between fiber optic and free space.
The invention may include any one of these separate aspects individually, or any combination of these separate aspects.